A significant amount of research and development has been undertaken in recent years towards environmental clean-up operations, and in particular to the purification and decontamination of groundwater, wastewater, and drinking water. A variety of techniques have been used in the prior art to destroy or remove contaminating and toxic materials such as trace organic and inorganic compounds; substances which produce color, taste and odor; pathogenic bacteria; and harmful suspended materials.
A technique is known in the art, usually under the name “electro-hydraulics”, which utilizes high-energy electrical discharge into a volume of liquid for the purpose of disinfecting water, changing chemical constituents and recovering metals and other substances from liquids or slurries (see, for example, U.S. Pat. No. 3,366,564 to Allen; U.S. Pat. No. 3,402,120 to Allen et al.; and U.S. Pat. No. 4,957,606 to Juvan). According to this technique, an electro-hydraulic shock wave within the liquid, intensive light radiation and thermo-chemical reactions are initiated by arc discharge into a spark gap formed by the electrodes immersed in the liquid.
FIG. 1 shows an electric scheme of a typical prior art system 10 for treatment of contaminated liquid by utilizing high-energy electrical discharge. The system 10 includes a high-voltage supply device 11 having a rectifier (not shown) coupled to a high voltage capacitor bank 12 that comprises one or more capacitors. The coupling of high-voltage supply device 11 to the capacitor bank 12 can, for example, be a direct “galvanic” connection. Alternatively, as is explained below, the connection can be through a resistive element and/or a switching element. The supply device 11 and the high voltage capacitor bank 12 form together a charge circuit A.
The system 10 also includes a high current switch 13 in series with the capacitor bank 12 and a pair of electrodes 14a and 14b separated by a gap in series with high current switch 13. In operation, the electrodes 14a and 14b are in contact with a liquid 15 for providing an electric discharge in the gap therebetween within the liquid. The capacitor bank 12, together with the high current switch 13, the electrodes 14a and 14b, and all interconnection cables therebetween form a discharge circuit B. For safety reasons, one of the terminals of the high-voltage supply device 11 (for example, which is connected to the electrode 14b) is permanently grounded. For example, only one of the electrodes (14a in FIG. 1) can be immersed in the liquid 15 under treatment, whereas the second electrode (14b in FIG. 1) can be coupled to or associated with a conductive body of the treatment vessel 16 holding the liquid 15. When desired, both electrodes can be immersed in the liquid 15 under treatment.
In operation, the capacitor bank 12 is charged by the voltage supply device 11. During the charging of the capacitor bank 12, the high current switch 13 is open. After the charging, the capacitor bank 12 can be discharged by closing the switch 13, in order to supply a high voltage to the electrodes 14a and 14b, and thereby generate an electric current pulse therebetween through the liquid under treatment. The closing of the high current switch 13 is usually activated by an ignition circuit (not shown) launching an ignition electric pulse to the switch 13.
Despite the apparent simplicity, the system 10 suffers from a number of limitations. In particular, the current charging the capacitor bank 12 has a form of an attenuated exponent. Accordingly, the charging current is large only at the very beginning of the charging process, and then the charging current becomes smaller over time. As a result, the power supply efficiency is low.
Another drawback is associated with the transient current behavior in the discharge circuit B. Since the discharge circuit B represents a series RLC circuit, the transient response of the circuit B depends on the damping factor ζ that is given by
  ζ  =            R      2        ⁢                  C        L            where C is the capacitance (in Farads) of the capacitor bank 12, L is the inductance (in Henrys) and R is the resistance (in Ohms) of the discharge circuit B.
The current behavior i(t) during a transient response for different ζ is shown in FIG. 2. As can be seen, this behavior depends on the value of ζ. In particular, when 0<ζ<1 (the under-damped response, curves 21-23), the transient current decays with oscillation. On the other hand, the transient current decays without oscillations occur when the ζ≧1 (the critically damped, curves 24; and over-damped response, curves 25-25).
In the case of oscillating current decays, the negative reverse components IR of the oscillating transient current i(t) produce a reverse voltage of high amplitude across the capacitor bank 12. As a result, the corresponding reverse discharge current can pass through the high-voltage supply device 11, thereby damaging it.
In order to decrease the reverse current of the electric discharge through the high-voltage supply device 11, a current limiting resistor 17 is usually included into this chain between the capacitor bank 12 and the voltage supply device 11 for limiting the discharge current. Although this provision enables protection of the voltage supply device 11 from damage, it results in electric losses and extra expenses.